1. Field of the Invention
The present invention relates to a display and a method of driving a display.
2. Description of the Related Art
In a display such as an organic electroluminescent (EL) display which controls the optical characteristics of each display element by a magnitude of a drive current passed through the display element, image quality deterioration such as luminance unevenness occurs if magnitudes of the drive currents vary. Therefore, when an active matrix driving method is used in this display, the characteristics of a drive transistor for controlling the magnitude of the drive current must be substantially the same between pixels. In this display, however, the drive transistors are normally formed on an insulator such as a glass substrate, so their characteristics easily vary.
U.S. Pat. No. 6,373,454 describes an organic EL display using a current mirror circuit in a pixel.
This pixel includes an n-channel field-effect transistor as the drive transistor, an organic EL element, and a capacitor. The source of the drive transistor is connected to a power supply line at a lower electric potential, and the capacitor is connected between the gate of the drive transistor and the power supply line. The anode of the organic EL element is connected to a power supply line at a higher electric potential.
The pixel circuit is driven as described below.
Firstly, the drain of the n-channel field-effect transistor is connected to its gate. A current Isig at a magnitude corresponding to a magnitude of a video signal is made to flow between the drain and source of the n-channel field-effect transistor. This operation sets the voltage between electrodes of the capacitor, equal to a gate-to-source voltage necessary for the n-channel field-effect transistor to pass the current Isig through its channel.
Then, the drain of the n-channel field-effect transistor is disconnected from its gate, and the voltage between the electrodes of the capacitor is maintained. The drain of the n-channel field-effect transistor is subsequently connected to the cathode of the organic EL element. This allows a drive current Idrv at a magnitude almost equal to that of the current Isig to flow through the organic EL element. The organic EL element emits light at a luminance corresponding to the magnitude of the drive current Idrv.
The above configuration makes it possible for the drive current Idrv, which flows between the drain and source of the n-channel field-effect transistor during a retention period following a write period, to have a magnitude almost equal to a magnitude of the current Isig supplied as a video signal during the write period. Therefore, the influence of not only a threshold value Vth but also the mobility, dimensions, and the like of the n-channel field-effect transistor on the drive current Idrv can be eliminated.
However, it is difficult for the above display to write the video signal Isig when a magnitude of the drive current Idrv corresponds to the video signal Isig is small. Therefore, the display unevenness easily occurs when an image of a low gray level is displayed. Consequently, it is difficult to achieve a contrast ratio as designed.